System and method of time-resolved, three-dimensional angiography

ABSTRACT

A method for generating time-resolved 3D medical images of a subject by imparting temporal information from a time-series of 2D medical images into 3D images of the subject. Generally speaking, this is achieved by acquired image data using a medical imaging system, generating a time-series of 2D images of a ROI from at least a portion of the acquired image data, reconstructing a 3D image substantially without temporal resolution from the acquired image data, and selectively combining the time series of 2D images with the 3D image. Selective combination typically involves registering frames of the time-series of 2D images with the 3D image, projecting pixel values from the 2D image frames “into” the 3D image, and weighting the 3D image with the projected pixel values for each frame of the time-series of 2D images. This method is particularly useful for generating 4D-DSA images, that is, time-resolved 3D-DSA images, from a time-series of 2D-DSA images acquired via single plane or biplane x-ray acquisitions with 3D images acquired via a rotational DSA acquisition. 4D-DSA images can also be generated by selectively combining a time-series of 2D-DSA images generated from individual projections from a rotational x-ray acquisition with a 3D image reconstructed from substantially all of the projection views acquired during the rotational x-ray acquisition. These DSA images may have a spatial resolution on the order of 512 3  pixels and a temporal resolution of about 30 frames per second, which represents an increase over traditional 3D-DSA frame rates by a factor between 150 and 600.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.12/542,376, filed Aug. 17, 2009, which is incorporated herein byreference in its entirety.

BACKGROUND OF THE INVENTION

The present invention is related to angiography and, in particular, theinvention relates to a system and method for producing time-resolved,three-dimensional (3D) angiographic images.

Since the introduction of angiography beginning with the direct carotidartery punctures of Moniz in 1927, there have been ongoing attempts todevelop angiographic techniques that provide diagnostic images of thevasculature, while simultaneously reducing the invasiveness associatedwith the procedure. For decades, post-processing of images was largelylimited to the use of film subtraction techniques. Initial angiographictechniques involved direct arterial punctures and the manipulation of aneedle through which a contrast medium was injected. These practiceswere associated with a significant incidence of serious complications.The development of percutaneous techniques allowing the use of a singlecatheter to study multiple arterial segments reduced, but this by nomeans eliminated, these adverse events. In the late 1970's, a techniqueknown as digital subtraction angiography (DSA) was developed based onreal-time digital processing equipment. Because of the advantages ofdigital processing, it was originally hoped that DSA could beconsistently implemented using an IV injection of contrast medium, thusreducing both the discomfort and the incidence of complicationsassociated with direct IA injections.

However, it quickly became apparent that the IV-DSA technique waslimited by problems due to suboptimal viewing angles and vessel overlapthat could only be reduced by repeated injections. Even then, thesefactors were problematic unless a projection that avoided the overlap ofrelevant vascular structures could be defined. Similar problems occurredwhen using biplane acquisitions. Also, because of the limited amount ofsignal associated with the IV injection of contrast medium, IV-DSA wasbest performed in conditions with adequate cardiac output and minimalpatient motion. IV-DSA was consequently replaced by techniques thatcombined similar digital processing with standard IA angiographicexaminations. Nevertheless, because DSA can significantly reduce boththe time necessary to perform an angiographic examination and the amountof contrast medium that was required, its availability resulted in asignificant reduction in the adverse events associated with angiography.Due to steady advancements in both hardware and software, DSA can nowprovide exquisite depictions of the vasculature in both 2D androtational 3D formats. Three-dimensional digital subtraction angiography(3D-DSA) has become an important component in the diagnosis andmanagement of people with a large variety of central nervous systemvascular diseases.

Current limitations in the temporal resolution capabilities of x-rayangiographic equipment require that rotational acquisitions be obtainedover a minimum time of about 5 seconds. Even with perfect timing of anacquisition so that arterial structures are fully opacified at the onsetof a rotation, there is almost always some filling of venous structuresby the end of the rotation. Display of a “pure” image of arterialanatomy is only achieved by thresholding such that venous structures,which contain lower concentrations of contrast medium than arterialstructures, are no longer apparent in the image. This limitation is asignificant factor in making it prohibitively difficult to accuratelymeasure the dimensions of both normal and abnormal vascular structures.Current DSA-based techniques do not depict the temporal sequence offilling in a reconstructed 3D-DSA volume.

In recent years competition for traditional DSA has emerged in the formof CT angiography (CTA) and Magnetic Resonance Angiography (MRA). CTAprovides high spatial resolution, but is not time-resolved unless theimaging volume is severely limited. CTA is also limited as a standalonediagnostic modality by artifacts caused by bone at the skull base andthe contamination of arterial images with opacified venous structures.Further, CTA provides no functionality for guiding or monitoringminimally-invasive endovascular interventions. Significant advances havebeen made in both the spatial and the temporal resolution qualities ofMRA. Currently, gadolinium-enhanced time-resolved MRA (TRICKS) is widelyviewed as a dominant clinical standard for time-resolved MRA. TRICKSenables voxel sizes of about 10 mm³ and a temporal resolution ofapproximately 10 seconds. Advancements such as HYBRID HYPR MRAtechniques, which violate the Nyquist theorem by factors approaching1000, can provide images with sub-millimeter isotropic resolution atframe times just under 1 second. Nonetheless, the spatial and temporalresolution of MRA are not adequate for all imaging situations and itscosts are considerable.

Shortcomings of existing angiography methods are particularly prevalentwhen imaging the small size and convoluted course of the intracranialvasculature. With traditional DSA it is difficult or impossible to imageand display these structures without the overlap of adjacent vessels.This problem is compounded when visualizing abnormal structures withcomplex geometry, such as aneurysms, or when abnormally fast or slowflow is present, such as in vascular malformations or ischemic strokes.As cerebrovascular diseases are increasingly treated using minimallyinvasive endovascular techniques where a physical is dependent uponimaging techniques for visualization of vascular structures, it isbecoming more important to develop imaging methods that allow cleardefinition of vascular anatomy and flow patterns. Such information isbecoming a prerequisite for both pre-treatment planning and the guidanceof interventional procedures. For example, the endovascular treatment ofvascular disease can require accurate navigation through the small andtortuous vessels of the brain and spinal cord. Currently this involvesthe use of roadmap that must be “reset” numerous times during a typicalprocedure. In fact, it is not uncommon to have 15 to 20 resets during agiven procedure. Not only does this use large amounts of contrastmedium, but the risk of thromboembolic complications increases with eachinjection.

It would therefore be desirable to have a system and method forproducing time-resolved, three-dimensional images of the vasculaturewith an improved spatial and temporal resolution over those possiblecurrently. The method would allow arterial vasculature to bedistinguished from venous vasculature, which would in turn allow the useof IV injections of contrast medium in cases where IA injections arecurrently performed. This would also allow 3D volumes to be viewed as adynamic sequence, allowing an improved understanding of vasculardiseases and providing a basis for more accurate and versatile roadmapsfor use in interventional procedures.

SUMMARY OF THE INVENTION

The present invention overcomes the aforementioned drawbacks byproviding a system and method for generating detailed series oftime-resolved, three-dimensional medical images of a subject, with bothhigh temporal resolution and excellent spatial resolution, by impartingtemporal information from a time-series of 2D images into a still 3Dimage. This method includes acquiring image data from a subject using amedical imaging system, generating a time-series of two-dimensionalimages from at least a portion of the acquired image data, andreconstructing a three-dimensional image substantially without temporalresolution from at least a portion of the acquired image data. Themethod also includes producing a time-resolved three-dimensional imageof the subject by selectively combining the three-dimensional imagesubstantially without temporal resolution and the time-series oftwo-dimensional images.

Another aspect of the present invention includes a method for producinga time-resolved three-dimensional image of a subject by acquiringtime-resolved image data from a region-of-interest (ROI) in the subjectin a first acquisition performed over a time period during which a bolusof contrast agent passes through the ROI and generating a time-series oftwo-dimensional images from image data acquired in the firstacquisition. The method also includes acquiring image data from the ROIin a second acquisition, reconstructing a three-dimensional imagesubstantially without temporal resolution from the image data acquiredin the second acquisition, and producing a time-resolvedthree-dimensional image of the subject by selectively combing thetime-series of two-dimensional images and the three-dimensional imagesubstantially without temporal resolution. In this method, the firstacquisition may be performed using a single plane or biplane x-raysystem.

In yet another aspect of the present invention, a method is provided forproducing a time-resolved three-dimensional image of a subject byacquiring projection views of a region-of-interest (ROI) in the subjectover a selected time period using a rotational acquisition, while abolus of contrast agent passes through the ROI during a portion of theselected time period. The method also includes generating a time-seriesof two-dimensional images of the ROI from projection views acquiredduring the portion of the selected time period during which the bolus ofcontrast agent passes through the ROI. The method further includesreconstructing a three-dimensional image of the ROI substantiallywithout temporal resolution from substantially all of the acquiredprojection views and producing the time-resolved three-dimensional imageof the subject by selectively combining the time-series oftwo-dimensional images and the three-dimensional image without temporalresolution

Various other features of the present invention will be made apparentfrom the following detailed description and the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B depict a rotational x-ray system configured to carry outa process in accordance with the present invention;

FIG. 2 is a flowchart setting forth the general steps for producing atime-resolved 3D image in accordance with the present invention;

FIG. 3 is a flowchart setting forth the steps for producing a 4D-DSAimage from time-resolved 2D images acquired using a single plane x-raysystem in accordance with the present invention;

FIG. 4 schematically depicts the selective combination of a 3D imagewith a 2D-DSA image frame acquired using a single plane x-ray system inaccordance with the present invention;

FIG. 5 is a flowchart setting forth the steps for producing a 4D-DSAimage from a pair of orthogonal, time-resolved 2D images acquired usinga biplane x-ray system in accordance with the present invention;

FIG. 6 schematically depicts the selective combination of a 3D imagewith a two orthogonal 2D-DSA image frames acquired using a biplane x-raysystem in accordance with the present invention;

FIG. 7 is a flowchart setting forth the steps for producing a 4D-DSAimage from a single set of projections views acquired using a rotationalx-ray system or CT system in accordance with the present invention;

FIG. 8 schematically depicts the selective combination of a 3D imagereconstructed from a full set of projection views with images generatedfrom a pair of individual projection views selected from the set inaccordance with the present invention; and

FIG. 9 depicts the combination of images involved in generating a 4Dfluoroscopic image.

GENERAL DESCRIPTION OF THE INVENTION

Referring to FIG. 1A, the present invention may employs a rotationalx-ray system that is designed specifically for use in connection withinterventional procedures. It is characterized by a gantry having aC-arm 10 which carries an x-ray source assembly 12 on one of its endsand an x-ray detector array assembly 14 at its other end. The gantryenables the x-ray source 12 and detector 14 to be oriented in differentpositions and angles around a patient disposed on a table 16, whileenabling a physician access to the patient.

The gantry includes an L-shaped pedestal 18 which has a horizontal leg20 that extends beneath the table 16 and a vertical leg 22 that extendsupward at the end of the horizontal leg 20 that is spaced from of thetable 16. A support arm 24 is rotatably fastened to the upper end ofvertical leg 22 for rotation about a horizontal pivot axis 26. The pivotaxis 26 is aligned with the centerline of the table 16 and the arm 24extends radially outward from the pivot axis 26 to support a C-arm driveassembly 27 on its outer end. The C-arm 10 is slidably fastened to thedrive assembly 27 and is coupled to a drive motor (not shown) whichslides the C-arm 10 to revolve it about a C-axis 28 as indicated byarrows 30. The pivot axis 26 and C-axis 28 intersect each other at anisocenter 36 located above the table 16 and they are perpendicular toeach other.

The x-ray source assembly 12 is mounted to one end of the C-arm 10 andthe detector array assembly 14 is mounted to its other end. The x-raysource 12 emits a beam of x-rays which are directed at the detectorarray 14. Both assemblies 12 and 14 extend radially inward to the pivotaxis 26 such that the center ray of this beam passes through the systemisocenter 36. The center ray of the beam can thus be rotated about thesystem isocenter around either the pivot axis 26 or the C-axis 28, orboth during the acquisition of x-ray attenuation data from a subjectplaced on the table 16.

The x-ray source assembly 12 contains an x-ray source which emits a beamof x-rays when energized. The center ray passes through the systemisocenter 36 and impinges on a two-dimensional flat panel digitaldetector housed in the detector assembly 14. The detector 38 is a 2048by 2048 element two-dimensional array of detector elements having a sizeof 41 cm by 41 cm. Each element produces an electrical signal thatrepresents the intensity of an impinging x-ray and hence the attenuationof the x-ray as it passes through the patient. During a scan the x-raysource assembly 12 and detector array assembly 14 are rotated about thesystem isocenter 36 to acquire x-ray attenuation projection data fromdifferent angles. The detector array is able to acquire 30 projections,or views, per second and this is the limiting factor that determines howmany views can be acquired for a prescribed scan path and speed.

Referring particularly to FIG. 1B, the rotation of the assemblies 12 and14 and the operation of the x-ray source are governed by a controlmechanism 40 of the x-ray system. The control mechanism 40 includes anx-ray controller 42 that provides power and timing signals to the x-raysource 32. A data acquisition system (DAS) 44 in the control mechanism40 samples data from detector elements 38 and passes the data to animage reconstructor 45. The image reconstructor 45, receives digitizedx-ray data from the DAS 44 and performs high speed image reconstructionaccording to the methods of the present invention. The reconstructedimage is applied as an input to a computer 46 which stores the image ina mass storage device 49 or processes the image further to produceparametric images according to the teachings of the present invention.It is contemplated that the computer 46 may be or include components ofa digital vascular image processor (DVIP) system.

The control mechanism 40 also includes gantry motor controller 47 and aC-axis motor controller 48. In response to motion commands from thecomputer 46 the motor controllers 47 and 48 provide power to motors inthe x-ray system that produce the rotations about respective pivot axis26 and C-axis 28. As will be discussed below, a program executed by thecomputer 46 generates motion commands to the motor drives 47 and 48 tomove the assemblies 12 and 14 in a prescribed scan path.

The computer 46 also receives commands and scanning parameters from anoperator via console 50 that has a keyboard and other manually operablecontrols. An associated cathode ray tube display 52 allows the operatorto observe the reconstructed image and other data from the computer 46.The operator supplied commands are used by the computer 46 under thedirection of stored programs to provide control signals and informationto the DAS 44, the x-ray controller 42 and the motor controllers 47 and48. In addition, computer 46 operates a table motor controller 54 whichcontrols the motorized table 16 to position the patient with respect tothe system isocenter 36.

Whereas conventional reconstruction methods generally necessitate theacquisition of a minimum number of projections dictated by the Nyquisttheorem, the present invention provides a fundamentally new method forimparting temporal resolution from a time-series of 2D images into 3Dimage volumes to create time-resolved 3D medical images. This allows,among other things, the production of 3D angiograms with both exquisitedetail and high temporal resolution. The method can be implemented usinga wide-variety of medical imaging systems, such as CT systems,fluoroscopy systems, and the above-discussed rotational x-ray system,either alone or in combination. Accordingly, the present descriptionfirst presents a generalized method for producing time-resolved 3Dimages before proceeding to more specific implementations and extensionsof the method.

Referring now to FIG. 2, a general method for producing a time-resolved3D image begins at process block 100 with the acquisition of image datafrom a region-of-interest in a subject using a medical imaging system,such as a CT system or a single-plane, biplane, or rotational x-raysystems. At process block 102, a time-series of 2D images is generatedfrom at least a portion of the acquired image data. While thetime-series of 2D images can have a high temporal and spatial resolutionand may include images acquired at different angles around the subject,it generally cannot provide a sophisticated 3D depiction of the subject.The production of the time-series of 2D images may be convolved with aconvolution kernel in order to provide local spatial coverage with adesired weighting. For example, these weighted images can provideinformation detailing how much of a vessel tree is present at a giventime. It is contemplated that this process can increase SNR by a factorof three over that provided by the original time series pixels whenusing a 3×3 convolution kernel. At process block 104, a 3D image of thesubject is reconstructed from the acquired image data. Though individualprojections used to reconstruct this 3D image may themselves convey somedegree of temporal information, the reconstructed 3D image itself issubstantially free of temporal resolution. For brevity, the 3D imagesubstantially without temporal resolution and the time-series of 2Dimages may simply be referred to as the “3D image” and “2D images,”respectively. It should be noted that the acquisition and reconstructionof the above sets of image data can be performed in accordance withconstrained reconstruction techniques, such as highly constrainedbackprojection reconstruction (HYPR), to improve SNR and permitpotential radiation and contrast agent dose reductions.

At process block 106, the time-series of 2D images and the static 3Dimage are selectively combined so that the temporal information includedin the 2D images is imparted into the 3D image. This results in theproduction of a time-resolved 3D image of the subject with high temporaland spatial resolution. While the selective combination process variesbased on the medical imaging system used and the nature of the acquiredimage data, it generally involves the steps of (1) registering the 2Dimages to the 3D image, (2) projecting the attenuation value of thepixels in the 2D images into the 3D image, and (3) weighting the 3Dimage with the projected values for each individual frame of thetime-series of 2D images. It is contemplated that the temporal weightingin step (3) generally involves multiplying the projected pixel valueswith the 3D image. These three steps, which can be referred to as“multiplicative projection processing” (MPP), may be accompanied byadditional steps to improve image quality or reduce the prevalence oferrors and artifacts. For example, the intensity values of pixels andvoxels in the 2D images and 3D image produced at process blocks 102 and104 may quantify an x-ray attenuation level at a given location in thesubject. These attenuation levels may not be preserved when multiplyingthe 3D image with projected pixel values. Accordingly, more accurateindications of the attenuation levels may be restored by taking a rootof the intensity value at each voxel in the time-resolved 3D image, forexample, by taking the n-th root if (n−1) different sets of 2D imagesare used to weight the 3D image. Other processing steps can be performedbefore the time-resolved 3D image is displayed at process block 108.

The 2D images and 3D image produced at process blocks 102 and 104,respectively, can be produced using DSA techniques. That is, 2D imagesdepicting the subject's vasculature can be produced by reconstructingimage data acquired as a bolus of contrast passes through the ROI andsubtracting out a pre-contrast, or “mask,” image acquired before theadministration of contrast agent. Likewise, a 3D image of the samevasculature can be produced by reconstructing image data acquired ascontrast agent occupies the ROI and subtracting out a mask image toremove signal associated with non-vascular structures. As will bediscussed below, depending on the imaging situation, the time series of2D-DSA images and the 3D-DSA images can be produced from image dataacquired using a single medical imaging system and contrast agentinjection or from different sets of image data acquired separately usingdifferent medical imaging systems and contrast agent injections. Ineither case, the time-resolved 3D image produced by combining the DSAimages depicts the subject's vasculature with both excellent spatial andexcellent temporal resolution and may thus be referred to as a 4D-DSAimage. In additional, the 4D-DSA images can be displayed as “pure”arterial, pure venous, or composite arterial and venous images and canbe fully rotated during each state of the filling of the vasculature,thereby enabling greatly simplified interpretation of vascular dynamics.The spatial resolution of these 4D-DSA images is generally on the orderof 512³ pixels at about 30 frames per second. This represents anincrease over traditional 3D-DSA frame rates by a factor between 150 and600, without any significant image quality penalty being incurred.

The acquisition of contrast enhanced image data can be performedfollowing the administration of contrast agent to the subject via eitherIV or IA injection. When scanning a local area, IA injections allow highimage quality and temporal resolution as well as reduced contrast agentdose. However, IV injections are often more suitable for scanning largerregions where multiple IA injections at different locations anddifferent arteries would otherwise be required. For example, there aremany clinical cases where multiple 3D-DSA acquisitions, each using adifferent IA injection, are performed to produce separate studies thatcan be merged into a larger high quality vascular tree. While separateIA acquisitions may be employed for generating the time-series of 2Dimages used by the present invention for temporal weighting, the use ofan intravenous injection for this purpose provides a mechanism forsimultaneously synchronized imparting temporal information to all of thepreviously acquired anatomical locations present in instances when thereare multiple, separate, IA 3D-DSA studies. This process reduces thelikelihood of complications associated with IA contrast agent injectionsand improves scan efficiency. Further, there is filling of arteries andveins with the same concentration of contrast medium in scans performedusing IV rather than an IA contrast agent injections, thus allowing thevisualization of venous and arterial structures at the same threshold.

Referring to FIG. 3, a more specific implementation of the above methodcan be employed to produce a 4D-DSA image of a subject using asingle-plane x-ray system in combination with a rotational x-ray systemor CT system. This particular method begins at process block 110, whentime-resolved image data from a ROI in the subject is acquired using thesingle-plane system following the administration of a contrast agent tothe subject. Using the above-discussed DSA techniques, a time-series of2D-DSA images at the selected angle is generated at process block 112.These 2D-DSA images depict the contrast agent passing through andenhancing arterial structures in the ROI. The 2D-DSA images aresubstantially free of signal from non-vascular structures, as well assignal from venous structures can be excluded due to the high temporalresolution of the 2D acquisition. At process 114, a separate scan isperformed following a second administration of contrast agent to thesubject using either the rotational fluoroscopy system or CT system toacquire 3D image data from the ROI. A 3D-DSA image is reconstructed atprocess block 116 from the acquired 3D image data. Typically, vascularstructures in the 3D-DSA image are substantially opacified due to theuse of contrast agent and the time necessary for data acquisition.

Referring now to FIGS. 3 and 4, the images produced thus far can now beselectively combined with the steps indicated generally at 117 toproduce a 4D-DSA image with the detailed 3D resolution of the 3D-DSAimage and the temporal resolution of the time-series of 2D-DSA images.In the exemplary depiction of the selective combination process providedin FIG. 4, a single frame of the time-series of 2D-DSA images 130includes two image regions having arterial signal 132, while the 3D-DSAimage 134 includes both arterial signal 136 and venous signal 138 and139. At process block 118, a frame of the 2D-DSA image 130 is registeredto the 3D-DSA image 132 at the selected angle and, at process block 120,the values of the pixels in the 2D-DSA frame are projected along a linepassing through each respective pixel in a direction perpendicular tothe plane of the 2D-DSA frame. The projection of pixels with arterialsignal 132 into the 3D-DSA image is indicated generally at 140. Forsimplicity, the projection of pixels in the 2D-DSA frame with nocontrast is not shown. At process block 122, the 3D-DSA image 134 isweighted by the values projected from the 2D-DSA frame 130 to producethe 4D-DSA image 142. Typically, this includes multiplying the projectedvalues with the voxels of the 3D image that they intersect. Theweighting process results in the preservation of the arterial signal 136and the exclusion, or “zeroing-out,” of undesired venous signal 139 inthe 4D-DSA image. In addition, the intensity value of the arterialsignal 132 in the 2D-DSA frame is imparted into the 3D arterial signalvolume 136, thereby allowing the changes in arterial signal over timecaptured by the 2D-DSA images to be characterized in the 4D-DSA image.At decision block 124, if all of the frames have yet to be processed,the process moves to the next frame of the time-series of 2D-DSA imagesat process block 126 and repeats the selective combination process 117.This cycle continues until, at decision block 126, it is determined thata 4D-DSA image has been generated for all relevant time frames. The4D-DSA image can thus be displayed at process block 128.

The venous signal 138 preserved in the 4D-DSA image 142 illustrates apotential problem when generating 4D images using only a singletime-series of 2D images acquired at a single angle. Signal from desiredstructures, such as the arterial signal 132, can inadvertently bedeposited in 3D voxels representing undesired structures, such as thevenous region 138. The unwanted structures can thus be preserved in the4D image as “shadow artifacts” when their signal lies along theprojected values of a desired structure in a dimension inadequatelycharacterized by the time-series of 2D images. This can result, forexample, in a 4D-DSA image in which desired arterial structures areobscured by undesired venous structures for some time frames. However,this will cause a temporary anomaly in the contrast vs. time course forthe vein. If the time frames of the 4D-DSA image are analyzed, thisanomaly can be recognized as inconsistent with the general waveform ofthe vein and the vein can be suppressed in the time frame where theprojected arterial signal is strong. Accordingly, temporal parameterssuch as mean transit time (MTT) or time-to-fractional-peak can becalculated for each voxel and this information can be used to clean upshadow artifacts. To assist an operator in identifying shadow artifactsand temporal irregularities, the temporal parameters can be color-codedand superimposed on the 4D-DSA image displayed at process block 128. Thetemporal parameters can also be exploited to infer information relatedto potential diffusion abnormalities in the absence of direct perfusioninformation from parenchymal signal.

Referring to FIG. 5, a method for producing 4D-DSA images that are lessprone to shadow artifacts begins at process block 150 with theacquisition of image data at a first angle and an orthogonal secondangle using, for example, a biplane fluoroscopy system. The image dataacquired at the orthogonal angles is then reconstructed at process block152 to produce a first and second time-series of 2D-DSA images, that is,one for each orthogonal angle. For brevity, these two-time-series of2D-DSA images may together be referred to as “orthogonal 2D-DSA images.”At process block 154, a second scan is performed using, for example, arotational fluoroscopy system or CT system to acquire 3D image data fromthe subject. At process block 156, the acquired 3D image data isreconstructed to produce a 3D-DSA image of the subject that issubstantially without temporal resolution. Again, both the 3D-DSA andorthogonal 2D-DSA images are acquired while a contrast agent isadministered to the subject and signal corresponding to non-vascularstructures is removed by subtracting out a mask image.

Referring now to FIGS. 5 and 6, the process of selectively combining theorthogonal 2D-DSA images and the 3D-DSA image is indicated generally at157 and is depicted schematically in FIG. 6. Specifically, a frame ofthe first time-series of 2D-DSA images 170 is registered to the 3D-DSAimage 172 at the first angle and a frame of the second time-series of2D-DSA images 174 is registered to the 3D-DSA image 172 at the secondangle. The two time-series of 2D-DSA images are thus registered into the3D space of the 3D-DSA image at orthogonal angles. At process block 160,the pixel intensities of each orthogonal 2D-DSA frame are projected intothe 3D-DSA image. The projection of arterial signal 176, that is, pixelshaving contrast, is indicated generally at 178. For simplicity, theprojections from pixels having no contrast are not depicted in FIG. 6.At process block 162, the projected values from both of the orthogonal2D-DSA frames are used to weight the 3D-DSA image and thus produce the4D-DSA frame 180. Regions of arterial signal 182 from the 3D-DSA imageare thus carried over to the 4D-DSA image, though their intensities aregenerally modulated based on the projected intensity of thecorresponding arterial signal 176 in both 2D-DSA frames. Regions ofvenous signal 184 and 186 in the 3D-DSA image are excluded, or “zeroedout,” from the 4D-DSA image since there is no corresponding projectedsignal from both 2D-DSA frames. The removal of arterial signal 184 isnotable, especially compared to the above-described single-plane 4D-DSAmethod. Even though the venous signal 184 lies along a projection frompixels with contrast in the 2D-DSA frame 170, it does not lie along aprojection from pixels with contrast in the, orthogonal, 2D-DSA frame174. Accordingly, the venous signal 184 is zeroed out by projectionsfrom pixels without contrast (not shown) in the 2D-DSA frame 174.

At decision block 164, if all the frames have not been processed, theprocess moves to the next time frame of both time-series of 2D-DSAimages at process block 166 and the selective combination process 157 isrepeated so that orthogonal projections weight the 3D-DSA image andenable formation of frames of the 4D-DSA image. This cycle continuesuntil, at decision block 164, it is determined that the 4D-DSA image hasbeen rendered for all relevant time frames. Following its completion,the 4D-DSA image can displayed at process block 128.

Referring to FIG. 7, while the above-described methods for generating4D-DSA images generally involve two separate scans using two separateimaging systems and contrast agent injections, the general method of thepresent invention, which was outlined with respect to FIG. 2, can beperformed on a single medical imaging system using a single contrastagent injection. Such a method begins at process block 200 with therotational acquisition of projection views of a ROI in a subject using arotational imaging system. A contrast agent is administered to thesubject so that it passes through the ROI during a portion of the timeperiod during which the acquisition is performed. At process block 202,a time-series of 2D-DSA images is generated from projection viewsacquired while contrast agent passed through the ROI. A 3D-DSA image ofthe subject substantially without temporal resolution is reconstructedfrom substantially all of the acquired projection views at process block204. Because many of the projection views are acquired after the bolusof contrast agent has passed through the ROI, the region's vasculatureis generally depicted as opacified in this 3D-DSA image.

The 2D-DSA images and the 3D-DSA image are selectively combined toproduce a 4D-DSA image, as indicated generally at 205. Because arotational acquisition is used, the time-series of 2D-DSA imagesincludes projection images, or “projections,” each acquired at a giventime and angle. Accordingly, a single time frame of the 4D-DSA image canbe produced by selectively combining the 3D-DSA image with a set ofselected projections acquired over a given angular range ΔΘ, so long asthe time difference between each of selected projections is relativelysmall. For example, for an acquisition in which 150 projection views areacquired over a 5 second time period, consecutive projection images areseparated by 1 degree and 30 ms. A frame of a 4D-DSA image with a 0.2second temporal resolution can be thus be generated by selectivelycombining the 3D-DSA image with consecutive projection images acquiredover 6 degree angular range.

Referring now to FIGS. 7 and 8, therefore, at process block 206 eachprojection in the set of selected projection images is registered withthe 3D image at an appropriate angle. At process block 208, the pixelintensities in each of these projection images is projected into the3D-DSA image. At process block 210, the projected pixel values are thenused to weight the 3D-image, generally via multiplication, and generatea time frame of the 4D-DSA image. At decision block 212, if all frameshave yet to be process, the method moves to the next set of selectedprojection views at process block 214, which correspond to the next timeframe of the 4D-DSA image, and the selective combination process 205 isrepeated. This cycle continues until, at decision block 212, it isdetermined that the 4D-DSA image has been rendered for all relevant timeframes. Following its completion, the 4D-DSA image can displayed atprocess block 216. The formation of a 4D-DSA image frame 220 byselectively combining two registered projections images 222 and 224 anda 3D-DSA image without time dependence 226 is schematically depicted inFIG. 8. Projected arterial signal 228 from the projection images 222 and224 weights arterial voxels 230 in the 3D-DSA image 226, while projectedsignal from pixels without contrast 232 nullifies venous voxels 234 inthe 3D-DSA image 226. The resulting 4D-DSA image frame 220, thus,includes weighted arterial signal 236, but does not include undesiredvenous signal, despite the fact the venous voxels 234 and arterialvoxels 230 of the 3D-DSA image are aligned for one of the projectionimages.

It should be noted that there are potential tradeoffs with this singlerotational acquisition approach. The ability to effectively cancelvenous structures using the multiplicative projection operationincreases with ΔΘ. However, temporal resolution decreases as ΔΘincreases. In additional, temporal resolution may be degraded relativeto the above-described single-plane and biplane methods, since thecontrast bolus may not be made as compact during the rotation because ofthe potential for artifacts related to inconsistent projection weightingfrom contrast variations. Accordingly, rotation acquisition settings maybe changed to reach a desired balance of temporal resolution andartery/vein overlap ambiguity.

Time-resolved 3D images produced in accordance with the presentinvention have significantly improved spatial and temporal resolutionover images produced using traditional methods and, thus, have greatutility in the diagnosis, pre-treatment planning, and post-treatmentassessment of complex vascular conditions. In addition, these imagesallow the implementation of time-resolved true 3D roadmaps for use inminimally invasive interventional procedures, thereby facilitating theimproved manipulation of surgical devices in complex vasculature. Inparticular, the present invention allows the implementation of 4Dfluoroscopy using real-time sequences of surgical device movementcombined with spatially and temporally selectable roadmaps obtained from4D-DSA images. For example, catheter information acquired via eithersingle plane or biplane fluoroscopy can be embedded within 4D-DSAvascular time frames to permit viewing at an arbitrary angle withoutfurther gantry movement. Catheter information acquired via real-timesingle projection subtracted fluoroscopy can likewise be superimposed on4D-DSA time frames that are registered at the gantry angle is adjusted.

Referring to FIG. 9, in the formation of a fluoroscopic image, it isimportant that the spatial resolution of the surgical device bemaintained. This can differ from the formation of 4D-DSA time frameswhere time-resolved 2D-DSA time frames are convolved and used to get alocal spatial estimate of temporal weighting. It is better to provide anunconvolved and isolated depiction of the surgical device information sothat resolution is maintained, though it should be noted that the lackof convolution can encumber image registration. The formation of imagessuitable for proper magnification and registration between a movingsurgical device, a catheter, and a 4D-DSA image frame is illustrated inFIG. 9. With the catheter mostly out of the field-of-view (FOV),fluoroscopic image 240 and 242 are obtained before and after contrastinjection, respectively. These images are subtracted to define an image244 of the vessel position in the fluoroscopic image series. Thecatheter is then advanced to obtain image 246, which is subtracted from240 to form the catheter-only image 248 that is added to a selected 4DDSA frame 250 to form the 4D fluoroscopic mage 252. For comparison, intraditional fluoroscopy, a fluoroscopic image 254 is formed bysubtracting the image 246 depicting the advanced catheter from thefluoroscopic image 242 obtained after contrast injection.

When embedding surgical device information acquired via biplanefluoroscopy in 4D-DSA images, the fluoroscopic images are acquired inorthogonal pairs and the above process can be performed for both images.In this case, objects in the resulting 4D fluoroscopic image 252 areregistered with the single projection bi-plane images 240-246.Generally, the first step in the registration process is the applicationof block matching with variable magnification and position in order toregister images 244 and 250 in lateral and coronal views. Then thecatheter image 248 is added in to form the 4D fluoroscopic image 252,which is registered to the traditional fluoroscopic image 254 usingblock matching. A range of horizontal and vertical magnifications aretypically searched during the registration procedure. For eachmagnification a spatial displaced block search can be used to minimizethe sum of absolute differences between 4D DSA time frames and displacedfluoroscopic time frames and the magnification and translation thatminimizes this sum may be chosen for registering the images. Toaccelerate the search procedure in fluoroscopic dose reductionalgorithms it is possible to employ an iterative block matchingtechnique that initially uses large blocks and then proceeds to smallerblocks.

The formation of catheter image, such as image 248, can be noisy due tothe multiplicative combination of noise from two biplane time-resolvedimages. Therefore, a noise reduction scheme may be implemented prior tothe combination of the catheter image and the 4D-DSA time frame. Forexample, a spatially adaptive linear filter may be used so that in eachelement of an image sub-region matrix, the direction of the catheter isdetermined by calculating the sum of the pixel value within the block asa test object similar in size to the catheter is rotated through theblock. The minimal sum is, thus, achieved when the test object has thesame orientation as the catheter and a linear convolution filter can beapplied along this direction to reduce catheter discontinuities causedby noise in acquired biplane images. Different grid sizes, test objectrotation angles, and translation scheduling can be used depending onimage quality and processing speed requirements. The size of the linearconvolution kernel can also be varied to achieve a desired balancebetween noise and discontinuity reduction and potential errors incatheter tip localization. Following the application of this spatiallyadaptive linear filter, a motion adaptive integration of fluoroscopicimage frames can also be used to reduce noise. Alternative spatiallyadaptive filters may also be used to improve the intelligibility ofcatheter images. For example, another approach is to scan the initialcatheter images point by point and look at the direction in which themaximum linear sum over some number of pixels occurs. The linear filtercan then operate in this direction, since it corresponds to the localorientation of the catheter. Depending on the size of the convolutionkernel, the search grid can be adjusted and multiple convolutions of thesame catheter segment may be performed.

A distinction should be considered when comparing the relative noiseproperties of the fluoroscopic and DSA applications. In the case ofgenerating 4D-DSA frames the anatomical details are supplied by the3D-image, for example, as acquired by a rotational DSA examination, andthe acquisition's individual projections or independently acquired2D-DSA images provide a local spatially averaged estimate of how muchthe vasculature is present at each point in time. This spatial averagereduces noise and it is not required that the spatial information of thetime dependent frames be maintained. In contrast, it is beneficial tomaintain the spatial resolution of the fluoroscopic images depicting thecatheter so that there is no intrinsic noise averaging except for thatimposed by the filters discussed above. It should also be noted that thecomputation time for generating 4D-DSA frames and generating 4Dfluoroscopic images can differ significantly. It is not necessary toreconstruct 4D-DSA images in real time, whereas fluoroscopic imagesshould be subjected to registration and noise averaging algorithms inreal time with minimal latency.

With a biplane fluoroscopy system, surgical device information fromorthogonal time-resolved image series is typically multiplied into abinarized version of the 3D rotational DSA voxels. Following projectionfrom one dimension there is uniform deposition of surgical device signalacross the vasculature depicted in the 3D rotational DSA voxels. Theintersection of this signal with corresponding signal from theorthogonal view carves out the 3D catheter voxels. A single plane systemcannot acquire this additional orthogonal view. However, an alternativeis to deposit all of the catheter signal in the center of the vesselsdepicted in a binary rotational DSA display. The maximum intensityprojection (MIP) through this data set at any point of time can then besuperimposed on the MIP through a corresponding 4D-DSA time frame, thusresulting in images that are roughly equivalent to those produced usingbiplane methods and the advancement of the surgical device is wellrepresented. This approach of constraining the catheter to the center ofa vessel in the direction not captured by the single plane acquisitiondoes not involve any significant disadvantage compared to traditionalfluoroscopic views, in which catheter position is unknown in onedirection.

For systems without biplane capabilities, the flexibility of roadmapselection provided by the 4D DSA time frames can additionally beexploited by superimposing the single plane fluoroscopy on the MIP ofthe 4D DSA time frame taken at a given gantry angle. This involvesregistration at just the current projection for each gantry angle.Because it is not necessary that orthogonal surgical device viewsintersect to form a 3D spatial catheter volume, registration is of lessimportance and the full image quality of live fluoroscopy is essentiallymaintained. It is contemplated that this gantry rotation mode offersimproved SNR, since it does not involve the multiplicative noise effectsthat occur when embedding biplane surgical device information into a4D-DSA image, as discussed above for establishing a 4D fluoroscopicvolume without gantry rotation.

The present invention has been described in terms of the preferredembodiment, and it should be appreciated that many equivalents,alternatives, variations, and modifications, aside from those expresslystated, are possible and within the scope of the invention. Therefore,the invention should not be limited to a particular describedembodiment.

The invention claimed is:
 1. A system for producing a time-resolvedthree-dimensional image of a subject, the system comprising: a computerprocessor configured to: receive image data acquired from the subjectusing a single rotation of a rotational medical imaging system over atime period, wherein a single contrast agent injection is administeredto the subject during a portion of the time period; generate atime-series of two-dimensional images from the image data, each of thetwo-dimensional images corresponding to a different time in the timeperiod and a different angle relative to the subject, wherein each ofthe two-dimensional images comprises pixel intensity information;reconstruct a three-dimensional image without temporal resolution fromthe image data; determine, for each of a plurality of thetwo-dimensional images, voxel weightings in the three-dimensional imagewithout temporal resolution by multiplying the voxels with the pixelintensity information of a two-dimensional image in the plurality; andproduce a time-resolved three-dimensional image of the subject byselectively combining the three-dimensional image substantially withouttemporal resolution and the time-series of two-dimensional images, thevoxel weightings being used to nullify one or more voxels from thethree-dimensional image without temporal resolution to produce thetime-resolved three-dimensional image.
 2. The system of claim 1, whereinthe nullified voxels in the time-resolved three-dimensional imagecorrespond to undesired vascular structures.
 3. The system of claim 2,wherein at least one of the nullified voxels corresponding to anundesired vascular structure is nullified by the voxel having differentweightings from one or more of the two-dimensional images at differentangles.
 4. The system of claim 3, wherein the computer processor isfurther configured to remove shadowing artifacts from the time-resolvedthree-dimensional image based on a temporal parameter associated witheach voxel of the time-resolved three-dimensional image comprising atleast one of a mean transit time (MTT) or time to fractional peak value.5. The system of claim 4, wherein the computer processor is furtherconfigured to superimpose a color coded display of the temporalparameter on at least one of the time-resolved three-dimensional imageand a blood volume image generated from the three-dimensional imagesubstantially without temporal resolution.
 6. The system of claim 1,wherein the computer processor is further configured to generate aroadmap for an interventional procedure based on the time-resolvedthree-dimensional image of the subject.
 7. The system of claim 6,wherein the computer processor is further configured to embed real-timesurgical device information within at least one selected time frame ofthe time-resolved three-dimensional image of the subject.
 8. The systemof claim 7, wherein the computer processor is further configured toembed surgical device information acquired at a first and second angleusing a biplane fluoroscopy system within corresponding angles of thetime-resolved three-dimensional image of the subject.
 9. The system ofclaim 8, wherein the computer processor is further configured to embed:surgical device information acquired at a first angle using asingle-plane fluoroscopy system within a corresponding angle of thetime-resolved three-dimensional image to produce a hybrid data set,wherein along a second angle that is thirty to ninety degrees relativeto the first angle surgical device information is embedded within acenter of vessels depicted in the time-resolved three-dimensional image;and superimpose a maximum intensity projection (MIP) through the hybriddata set on a MIP through the time-resolved three-dimensional imagesubject at a corresponding angle.
 10. A non-transitory computer-readablemedium having computer-readable instructions stored thereon that whenexecuted by a computer system cause the computer system to perform amethod, the method comprising: receiving time-resolved image dataacquired from a region-of-interest (ROI) in the subject in a firstacquisition performed over a time period prior to and during which abolus of contrast agent passes through the ROI; generating a time-seriesof two-dimensional images from image data acquired in the firstacquisition, each of the two-dimensional images corresponding to adifferent time in the time period and a different angle relative to thesubject, wherein each of the two-dimensional images comprises pixelintensity information; receiving image data acquired from the ROI in asecond acquisition; reconstruct a three-dimensional image substantiallywithout temporal resolution from the image data acquired in the secondacquisition; determining, for each of a plurality of the two-dimensionalimages, voxel weightings in the three-dimensional image without temporalresolution by multiplying the voxels with the pixel intensityinformation of a two-dimensional image in the plurality; and producing atime-resolved three-dimensional image of the subject by selectivelycombining the time-series of two-dimensional images and thethree-dimensional image substantially without temporal resolution, thevoxel weightings being used to nullify one or more voxels from thethree-dimensional image without temporal resolution to produce thetime-resolved three-dimensional image.
 11. The computer-readable mediumof claim 10, wherein the method further comprises: registering thereconstructed three-dimensional image substantially without temporalresolution to the time-series of two-dimensional images; and projectinga value of each pixel in the time-series of two-dimensional images alonga line extending through each respective pixel in a directionperpendicular to a plane of the time-series of two-dimensional images.12. The computer-readable medium of claim 11, wherein the nullifiedvoxels in the time-resolved three-dimensional image correspond toundesired vascular structures; and wherein the nullified voxels in thetime-resolved three-dimensional image correspond to undesired vascularstructures in the ROI.
 13. The computer-readable medium of claim 10,wherein the method further comprises: receiving time-resolvedtwo-dimensional image data acquired at a first angle and a second anglebetween thirty and ninety degrees relative to the first angle using abiplane fluoroscopy system; and reconstructing a first and secondtime-series of two-dimensional images from the image data acquired atthe first and second angles, respectively.
 14. The computer-readablemedium of claim 10, wherein the image data acquired in the secondacquisition is acquired using a rotational x-ray system following anadministration of a contrast agent using at least one of intravenousinjection and intra-arterial injection.
 15. The computer-readable mediumof claim 10, wherein the method further comprises generating a roadmapfor an interventional procedure based on the time-resolvedthree-dimensional image of the subject.
 16. A system for producing atime-resolved three-dimensional image of a subject, the systemcomprising: a computer processor configured to: receive projection viewsof a region-of-interest (ROI) in the subject acquire over a selectedtime period using a single rotational acquisition, wherein a singlebolus of contrast agent passes through the ROI during a portion of theselected time period; generate a time-series of two-dimensional imagesof the ROI from projection views acquired during the portion of theselected time period during which the bolus of contrast agent passesthrough the ROI, each of the two-dimensional images corresponding to adifferent time in the time period and a different angle relative to thesubject, wherein each of the two-dimensional images comprises pixelintensity information; reconstruct a three-dimensional image of the ROIsubstantially without temporal resolution from substantially all of theacquired projection views; determine, for each of a plurality of thetwo-dimensional images, voxel weightings in the three-dimensional imagewithout temporal resolution by multiplying the voxels with the pixelintensity information of a two-dimensional image in the plurality; andproduce the time-resolved three-dimensional image of the subject byselectively combining the time-series of two-dimensional images and thethree-dimensional image without temporal resolution, the voxelweightings being used to nullify one or more voxels from thethree-dimensional image without temporal resolution to produce thetime-resolved three-dimensional image.
 17. The system of claim 16,wherein the computer processor is further configured to: register thereconstructed three-dimensional image substantially without temporalresolution to the time-series of two-dimensional images; and project avalue of each pixel in the time-series of two-dimensional images along aline extending through each respective pixel in a directionperpendicular to a plane of the time-series of two-dimensional images.18. The system of claim 17, wherein the nullified voxels in thetime-resolved three-dimensional image corresponding to undesiredvascular structures in the ROI.
 19. The system of claim 16, wherein thecomputer processor is configured to remove non-vascular structures fromthe time-series of two-dimensional images and the three-dimensionalimage substantially without temporal resolution by subtracting imagedata acquired from the ROI prior to passage of the contrast agent bolusthrough the ROI from the acquired projection views.
 20. The system ofclaim 16, wherein the projection views acquired during the portion ofthe selected time period during which the bolus of contrast agent passesthrough the ROI are acquired over a limited angular range of therotational acquisition.